Preemptive dynamic frequency selection

ABSTRACT

Briefly, in accordance with one embodiment of the invention, an access point may preemptively broadcast an alternate channel to switch to, along with an indication of the beacon timing for the alternate channel, prior to any catastrophic interference. The access point may switch to the alternate channel in the event of interference on the original channel without attempting to broadcast the alternate channel during the interference event. A mobile user may then know in advance of the interference event which alternate channel the access point switched to and may switch to the alternate channel.

BACKGROUND OF THE INVENTION

A wireless local area network (WLAN) system may employ dynamic frequencyselection (DFS) to select the best channel on which to operate,typically to avoid an interference event or interferers which may beother WLAN systems or unrelated devices emitting RF energy, or to avoidinterfering with other devices such as radars. Such a system may beprimarily intended to avoid interfering with other devices, particularlyradars, in order to meet regulatory requirements. However, such systemsdo not perform well in response to a catastrophic interferer. Acatastrophic interferer may be defined as one that causes a significantor total reduction in available throughput. A catastrophic interferermay be a non-WLAN device, or a WLAN device using an extremely aggressivechannel access mechanism. Such channel access mechanisms may be employedby a centrally controlled WLAN system in which an access point (AP) mayassume that it has complete control of the channel and which may notdefer its transmissions to other transmitters if it finds the channelbusy.

DESCRIPTION OF THE DRAWING FIGURES

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a block diagram of a wireless local area network communicationsystem in accordance with one embodiment of the present invention;

FIG. 2 is a timing diagram of dynamic frequency selection in a wirelessLAN system in accordance with one embodiment of the invention;

FIG. 3 is a flow diagram of a method for preemptive dynamic frequencyselection in accordance with one embodiment of the present invention;and

FIG. 4 is a diagram of a channel switch announcement message inaccordance with an embodiment of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals have been repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

Some portions of the detailed description that follows are presented interms of algorithms and symbolic representations of operations on databits or binary digital signals within a computer memory. Thesealgorithmic descriptions and representations may be the techniques usedby those skilled in the data processing arts to convey the substance oftheir work to others skilled in the art.

An algorithm is here, and generally, considered to be a self-consistentsequence of acts or operations leading to a desired result. Theseinclude physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. It has proven convenient at times,principally for reasons of common usage, to refer to these signals asbits, values, elements, symbols, characters, terms, numbers or the like.It should be understood, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as processing, computing, calculating,determining, or the like, refer to the action or processes of a computeror computing system, or similar electronic computing device or platform,that manipulate or transform data represented as physical, such aselectronic, quantities within the registers or memories of the computingplatform into other data similarly represented as physical quantitieswithin the memories, registers or other such information storage,transmission or display devices of the computing platform.

Embodiments of the present invention may include apparatuses forperforming the operations herein. This apparatus may be speciallyconstructed for the desired purposes, or it may comprise a generalpurpose computing device selectively activated or reconfigured by aprogram stored in the device. Such a program may be stored on a storagemedium, such as, but is not limited to, any type of disk includingfloppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-onlymemories (ROMs), random access memories (RAMs), electricallyprogrammable read-only memories (EPROMs), electrically erasable andprogrammable read only memories (EEPROMs), flash memory, magnetic oroptical cards, or any other type of media suitable for storingelectronic instructions, and capable of being coupled to a system busfor a computing platform.

The processes and displays presented herein are not inherently relatedto any particular computing device or other apparatus. Various generalpurpose systems may be used with programs in accordance with theteachings herein, or it may prove convenient to construct a morespecialized apparatus to perform the desired method. The desiredstructure for a variety of these systems will appear from thedescription below. In addition, embodiments of the present invention arenot described with reference to any particular programming language. Itwill be appreciated that a variety of programming languages may be usedto implement the teachings of the invention as described herein.

In the following description and claims, the terms coupled andconnected, along with their derivatives, may be used. In particularembodiments, connected may be used to indicate that two or more elementsare in direct physical or electrical contact with each other. Coupledmay mean that two or more elements are in direct physical or electricalcontact. However, coupled may also mean that two or more elements maynot be in direct contact with each other, but yet may still cooperate orinteract with each other.

It should be understood that embodiments of the present invention may beused in a variety of applications. Although the present invention is notlimited in this respect, the circuits disclosed herein may be used inmany apparatuses such as in the transmitters and receivers of a radiosystem. Radio systems intended to be included within the scope of thepresent invention include, by way of example only, wireless local areanetworks (WLAN) devices and wireless wide area network (WWAN) devicesincluding wireless network interface devices and network interface cards(NICs), base stations, access points (APs), gateways, bridges, hubs,cellular radiotelephone communication systems, satellite communicationsystems, two-way radio communication systems, one-way pagers, two-waypagers, personal communication systems (PCS), personal computers (PCs),personal digital assistants (PDAs), and the like, although the scope ofthe invention is not limited in this respect.

Types of wireless communication systems intended to be within the scopeof the present invention include, although are not limited to, WirelessLocal Area Network (WLAN), Wireless Wide Area Network (WWAN), CodeDivision Multiple Access (CDMA) cellular radiotelephone communicationsystems, Global System for Mobile Communications (GSM) cellularradiotelephone systems, North American Digital Cellular (NADC) cellularradiotelephone systems, Time Division Multiple Access (TDMA) systems,Extended-TDMA (E-TDMA) cellular radiotelephone systems, third generation(3G) systems like Wide-band CDMA (WCDMA), CDMA-2000, and the like,although the scope of the invention is not limited in this respect.

Referring now to FIG. 1, a wireless local area network communicationsystem in accordance with one embodiment of the present invention willbe discussed. In the WLAN communications system 100 shown in FIG. 1, amobile unit 110 may include a wireless transceiver 112 to couple to atleast one antenna 118 and to a processor 114 to provide baseband andmedia access control (MAC) processing functions. Processor 114 in oneembodiment may comprise a single processor, or alternatively maycomprise a baseband processor and an applications processor, althoughthe scope of the invention is not limited in this respect. Processor 114may couple to a memory 116 which may include volatile memory such asDRAM, non-volatile memory such as flash memory, or alternatively mayinclude other types of storage such as a hard disk drive, although thescope of the invention is not limited in this respect. Some portion orall of memory 116 may be included on the same integrated circuit asprocessor 114, or alternatively some portion or all of memory 116 may bedisposed on an integrated circuit or other medium, for example a harddisk drive, that is external to the integrated circuit of processor 114,although the scope of the invention is not limited in this respect.

Mobile unit 110 may communicate with access point 122 via wirelesscommunication link 132, where access point 122 may include at least oneantenna 120. In an alternative embodiment, access point 122 andoptionally mobile unit 110 may include two or more antennas, for exampleto provide a spatial division multiple access (SDMA) system or amultiple input, multiple output (MIMO) system, although the scope of theinvention is not limited in this respect. Access point 122 may couplewith network 130 so that mobile unit 110 may communicate with network130, including devices coupled to network 130, by communicating withaccess point 122 via wireless communication link 132. Network 130 mayinclude a public network such as a telephone network or the Internet, oralternatively network 130 may include a private network such as anintranet, or a combination of a public and a private network, althoughthe scope of the invention is not limited in this respect. Communicationbetween mobile unit 110 and access point 122 may be implemented via awireless local area network (WLAN), for example a network compliant witha an Institute of Electrical and Electronics Engineers (IEEE) standardsuch as IEEE 802.11a, IEEE 802.11b, HiperLAN-II, and so on, although thescope of the invention is not limited in this respect. In anotherembodiment, communication between mobile unit 110 and access point 122may be at least partially implemented via a cellular communicationnetwork compliant with a 3GPP standard, although the scope of theinvention is not limited in this respect.

Referring now to FIG. 2, a timing diagram of dynamic frequency selectionin a wireless LAN system in accordance with one embodiment of theinvention will be discussed. As shown in FIG. 2, channel A beacontransmissions are shown at 210 and channel B beacon transmissions areshown at 212. While operating on channel A, access point 122 maydetermine an alternate channel to which it might switch if channel Abecomes unsuitable, for example channel B. Access point 122 may transmitthe alternate channel, channel B, to mobile unit 110 and an offset time,t_(oB), in beacon transmissions 210 on channel A. Thus, in oneembodiment of the invention, access point 122 may preemptively announceto mobile unit 110 a predetermined alternative channel to which accesspoint 122 will switch prior to an actual catastrophic event, althoughthe scope of the invention is not limited in this respect. Mobile unit110 associated with access point 122 may perform background scanning ofother channels for potential alternate access points. In addition,however, mobile unit 110 may check the indicated alternate channel,channel B, at a time encompassing the indicated beacon offset, t_(oB),such that if access point 122 switches to the alternate channel, channelB, due to a catastrophic interferer, mobile unit 110 may readily detectthe switch.

The messages in beacon frames broadcast by access point 122 in channel Aindicating the alternate channel and an offset time of the beaconsbetween channels is shown at 214. In the event of the appearance of asevere interferer on channel A as shown at 216, access point 122 may beunable to transmit a channel switch announcement due to the presence ofthe catastrophic interferer at 216. As a result, access point 122 mayswitch to the previously indicated, predetermined, alternate channel,channel B, and resume transmitting beacons in the alternate channel atthe previously indicated offset time as shown at 218. Access point 122may switch to the predetermined, alternate channel without making anannouncement to mobile unit that the switch is occurring or hasoccurred. In one embodiment of the invention, the beacons transmitted byaccess point 122 in the new channel, channel B, may occur at a timeequal to the time of the last clearly transmitted beacon in channel A,plus the offset time, t_(oB) as shown in FIG. 2, although the scope ofthe invention is not limited in this respect. Upon switching to the newchannel, in this example channel B, access point 122 may transmitbeacons as shown at 212 with the same period at which the beacons inprevious channel were transmitted, but at a target beacon transmissiontime (TBTT) which may have an offset from the TBTT of beacons in theprevious channel by a time equal to the beacon offset time. In oneembodiment of the invention, mobile unit 110 may periodically switch tothe predetermined, alternate channel at the TBTT of the alternatechannel until beacons transmitted from access point 122 are detected onthe alternate channel, although the scope of the invention is notlimited in this respect.

Mobile unit 110 may detect the beacon transmitted by access point 122 inthe alternate channel, channel B, at 220 since access point 122transmitted the alternate channel and the offset time in channel Abeacons. In one embodiment of the invention, mobile unit 110 maycontinually background monitor the alternate channel at thepredetermined time as indicated by the offset time during a specifiedtime window to determine whether access point 122 has switched to thealternate channel. In the event mobile unit 110 detects that accesspoint has in fact switched to the alternate channel by detecting abeacon transmitted from access point 122 in the alternate, predeterminedchannel such as shown at 218, mobile unit 110 may likewise switch to thealternate channel, channel B, as shown at 222 to continue to communicatewith access point 122 and complete a transition to channel B, althoughthe scope of the invention is not limited in this respect. In analternative embodiment, access point may preemptively transmit multiplealternate channels and optionally multiple associated offset times forthe multiple alternate channels to accommodate a situation in whichthere may be a catastrophic interferer on one alternate channelsimultaneous with a catastrophic interferer on the original channel.Mobile unit 110 may check each alternate channel until a beacontransmitted by access point 122 is detected on an alternate channel, andmay optionally do so in an order preannounced by access point 122,although the scope of the invention is not limited in this respect. Inone such embodiment, the time offset from a first channel to asubsequent channel may be constant valued between the channels, and in afurther embodiment, the TBTT offset times for the alternate channels maybe ordered according to a predetermined order of alternate channelswhere the predetermined order may be based on, for example, the signalquality on the alternate channels, although the scope of the inventionis not limited in this respect.

In one embodiment of the invention, the beacon offset time t_(oB) may bechosen to be slightly shorter than one beacon interval. As a result, themaximum bounded time for a complete channel switch from channel A tochannel B may be slightly greater than one beacon interval, for exampleon the order of 100 ms, although the scope of the invention is notlimited in this respect. In a particular embodiment of the invention,such an arrangement may be sufficiently short that streaming mediaapplications will be unaffected by the channel switch, showing noglitches or other user- perceptible artifacts due to data loss, andinteractive services such as voice will not show any significantperceptual losses, although the scope of the invention is not limited inthis respect. As a result, real time streaming or interactiveapplications may be unaffected or relatively unaffected in the face ofsudden and severe interference, with no perceptible interruption to theuser, although the scope of the invention is not limited in thisrespect.

Referring now to FIG. 3, a flow diagram of a method for preemptivedynamic frequency selection in accordance with one embodiment of theinvention will be discussed. As shown in FIG. 3, mobile unit 110 maywait for a beacon transmitted from access point 122 at block 310, and onreceipt of a beacon, a determination may be made at block 312 whetherthe beacon contains a preemptive channel switch announcement message,which may include a predetermined, alternative channel and a beacon timeoffset. In the event the beacon contains a preemptive channel switchannouncement message, mobile unit 110 may update stored parameters thatindicated the predetermined, alternate channel and beacon offset time atblock 314. In the event the beacon does not contain a preemptive channelswitch announcement message, the mobile unit may continue to wait forsubsequent beacons at block 310. Mobile unit 110 may scan, optionallyindependently, the predetermined, alternative channel determined atblock 312 at the specified time at block 316 as determined by the beaconoffset time. A determination may be made at block 318 whether a beaconfrom the current access point 122 is detected in the predeterminedalternative channel at the time determined by the beacon offset time. Inthe event mobile unit 110 detects a beacon from the currently associatedaccess point 122 in the predetermined, alternative channel, mobile unit110 performs a channel switch to communicate with access point 122 inthe predetermined, alternate channel. After switching to thepredetermined alternate channel, mobile unit 110 may continue at blocks310 and 316, although the scope of the invention is not limited in thisrespect.

In one embodiment of the invention, wireless LAN communication system100 implementing an audio, video, or other real time system such as avoice or videoconference system may be arranged such that a boundedmaximum interrupt time cause by a severely interfered environment on theoriginal channel, such as shown at 216, may be tolerated by includingreceiver side buffering without interruption of the real time data asperceived by a user. In a further embodiment, transmitter side bufferingmay likewise be tolerant to the bounded maximum interrupt time to avoidtransmit buffer overflow. Such an audio, video, or other real timesystem may be arranged such that a bounded maximum interruption timecause by a severely interfered environment on the original channel, suchas shown at 216, may be tolerated without a drop in communication.Furthermore, a perceptual mitigation scheme may be utilized such that inthe event of severe interference, a temporary interruption of thetransmission of data may result in little or no perceptual effects asexperienced by a user, although the scope of the invention is notlimited in this respect.

Referring now to FIG. 4, a diagram of a channel switch announcementmessage in accordance with one embodiment of the invention will bediscussed. As shown in FIG. 4, channel switch announcement message 400may include an element identifier 410, an element 412 indicating thelength of the payload in bytes, an organizationally unique identifier(OUI) 414, a preemptive channel switch announcement sub-identifier 416,a channel number element 418 indicating a predetermined, alternatechannel, and a beacon offset time element 420 which may indicate abeacon offset time, for example in units of 32 microseconds, althoughthe scope of the invention is not limited in this respect. In oneparticular embodiment of the invention, channel switch announcementmessage 400 may be in the form of an information element as defined inthe IEEE 802.11-1997 standard, although the scope of the invention isnot limited in this respect. Such an information element may be uniquelyidentified as being a proprietary extended function, for example byusing an organizationally unique identifier (OUI) assigned by the IEEE.Such a message may be included in beacon frames transmitted by accesspoint 122, and may be interpreted by clients such as mobile unit 1100that support the extended functionality in accordance with the presentinvention, and ignored by clients that do not, although the scope of theinvention is not limited in this respect. In one particular embodiment,the invention may be implemented as an extension to any IEEE standards,for example the IEEE 802.11e standard or the IEEE 802.11h standard,although the scope of the invention is not limited in this respect.

Although the invention has been described with a certain degree ofparticularity, it should be recognized that elements thereof may bealtered by persons skilled in the art without departing from the spiritand scope of the invention. It is believed that the preemptive dynamicfrequency selection of the present invention and many of its attendantadvantages will be understood by the forgoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components thereof without departing from thescope and spirit of the invention or without sacrificing all of itsmaterial advantages, the form herein before described being merely anexplanatory embodiment thereof, and further without providingsubstantial change thereto. It is the intention of the claims toencompass and include such changes.

1. A method, comprising: transmitting on a first channel bothindications of a plurality of alternate channels and a plurality ofassociated time indications of transmissions on the alternate channels;and in response to an interference event on the first channel, scanningthe alternate channels in a preannounced order until a beacon isdetected on an alternate channel and switching to the alternate channel,wherein the indications of the alternate channels and the associatedtime indications are transmitted prior to the interference event.
 2. Amethod as claimed in claim 1, wherein said switching occurs withouttransmitting an indication of the alternate channel after detecting theinterference event.
 3. A method as claimed in claim 1, wherein saidassociated time indications include offset times arranged according to apredetermined order.
 4. A method as claimed in claim 1, wherein saidtransmitting on the first channel includes transmitting an indication ofthe alternate channel in an access point beacon on the first channel andwherein said time indication includes a time offset between access pointbeacons in the first channel and access point beacons in the alternatechannel.
 5. A method as claimed in claim 1, wherein said transmitting onthe first channel includes transmitting an indication of the alternatechannel in an access point beacon on the first channel and wherein saidtime indication includes a time offset between access point beacons inthe first channel and access point beacons in the alternate channel,wherein the time offset is less than a beacon interval.
 6. A method,comprising: receiving on a first channel both indications of a pluralityof alternate channels and a plurality of associated time indications oftransmissions on the alternate channels; and in the event ofinterference on the first channel, scanning the alternate channels in apreannounced order until a beacon is detected on an alternate channeland switching to the alternate channel, wherein the indications of thealternate channels and the associated time indications are transmittedprior to the interference event.
 7. A method as claimed in claim 6,wherein said switching occurs without transmitting an indication of thealternate channel after detecting the interference event.
 8. A method asclaimed in claim 6, further comprising listening for communication onthe alternate channel at a time based on the time indication.
 9. Amethod as claimed in claim 6, wherein said receiving occurs during anaccess point beacon on the first channel, and further comprisinglistening for an access point beacon on the alternate channel at a timebased on the time indication, and in the event an access point beacon isdetected on the alternate channel, switching to the alternate channel.10. A method as claimed in claim 6, wherein said transmitting receivingoccurs during an access point beacon on the first channel, and furthercomprising listening for an access point beacon on the alternate channelat a time based on the time indication, and in the event an access pointbeacon is detected on the alternate channel, switching to the alternatechannel, wherein the time indication includes an offset between anaccess point beacon on the first channel and an access point beacon onthe alternate channel.
 11. An article, comprising: a storage mediumhaving stored thereon instructions that, when executed by a computingplatform, result in dynamic frequency selection by: transmitting on afirst channel indications of a plurality of alternate channels and aplurality of associated time indications of transmissions on thealternate channels; and in response to an interference event on thefirst channel, scanning the alternate channels in a preannounced orderuntil a beacon is detected on an alternate channel and switching to thealternate channel, wherein the indications of the alternate channel andthe associated time indications are transmitted prior to theinterference event.
 12. An article as claimed in claim 11, wherein theswitching occurs without transmitting an indication of the alternatechannel after detecting the interference event.
 13. An article asclaimed in claim 11, wherein said associated time indications includeoffset times arranged according to a predetermined order.
 14. An articleas claimed in claim 11, wherein said transmitting on the first channelincludes transmitting an indication of the alternate channel in anaccess point beacon on the first channel and wherein said timeindication includes a time offset between access point beacons in thefirst channel and access point beacons in the alternate channel.
 15. Anarticle as claimed in claim 11, wherein said transmitting on the firstchannel includes transmitting an indication of the alternate channel inan access point beacon on the first channel and wherein said timeindication includes a time offset between access point beacons in thefirst channel and access point beacons in the alternate channel, whereinthe time offset is less than a beacon interval.
 16. An article,comprising: a storage medium having stored thereon instructions that,when executed by a computing platform, result in dynamic frequencyselection by: receiving on a first channel both indications of aplurality of alternate channels and a plurality of associated timeindications of transmissions on the alternate channels; and in the eventof interference on the first channel, scanning the alternate channels ina preannounced order until a beacon is detected on an alternate channeland switching to the alternate channel, wherein the indications of thealternate channels and the associated time indications are transmittedprior to the interference event.
 17. An article as claimed in claim 16,wherein the switching occurs without transmitting an indication of thealternate channel after detecting the interference event.
 18. An articleas claimed in claim 16, wherein the instructions, when executed, furtherresult in dynamic frequency selection by listening for communication onthe alternate channel at a time based on the time indication.
 19. Anarticle as claimed in claim 16, wherein said transmitting receivingoccurs during an access point beacon on the first channel, and whereinthe instructions, when executed, further result in dynamic frequencyselection by listening for an access point beacon on the alternatechannel at a time based on the time indication, and in the event anaccess point beacon is detected on the alternate channel, switching tothe alternate channel.
 20. An article as claimed in claim 16, whereinsaid transmitting receiving occurs during an access point beacon on thefirst channel, and wherein the instructions, when executed, furtherresult in dynamic frequency selection by listening for an access pointbeacon on the alternate channel at a time based on the time indication,and in the event an access point beacon is detected on the alternatechannel, switching to the alternate channel, wherein the time indicationincludes an offset between an access point beacon on the first channeland an access point beacon on the alternate channel.
 21. An apparatuscomprising: A transceiver; An omnidirectional antenna to couple to saidtransceiver; and A baseband processor to cause said transceiver totransmit on a first channel both indications of a plurality of alternatechannels and a plurality of associated time indications of transmissionson the alternate channels, and in response to an interference event onthe first channel, to scan the alternate channels in a preannouced orderuntil a beacon is detected on an alternate channel and switch to analternate channel, wherein the transceiver transmits the indications ofthe alternate channels, an order to check the alternate channels for abeacon, and the time indications prior to the interference event.
 22. Anapparatus as claimed in claim 21, said baseband processor to cause saidtransceiver to transmit on the first channel the indications of thealternate channels in an access point beacon on the first channel andwherein said time indications include a time offset between access pointbeacons in the first channel and access point beacons in the alternatechannels.
 23. An apparatus, comprising: a transceiver; anomnidirectional antenna to couple to said transceiver; and a basebandprocessor to cause said transceiver to receive on a first channel bothindications of a plurality of alternate channels and a plurality ofassociated time indications of transmissions on the alternate channels,and in the event of interference on the first channel, to scan thealternate channels in a preannounced order until a beacon is detected onan alternate channel and to switch to the alternate channel, wherein thetransceiver receives the indication of the alternate channel and thetime indication prior to the interference event.
 24. An apparatus asclaimed in claim 23, said baseband processor to cause said transceiverto receive an access point beacon on the first channel, the access pointbeacon including the time indications of transmissions on the alternatechannels, said baseband processor to further cause said transceiver tolisten for an access point beacon on an alternate channel at a timebased on a time indication according to the preannounced order, and inthe event an access point beacon is detected on the alternate channel,to switch to the alternate channel.